1. Field of the Invention
The present invention relates to a method for manufacturing single crystal ceramic powder, and single crystal ceramic powder, composite material and electronic elements using the single crystal ceramic powder.
2. Description of Related Art
Ceramic powder has been used for many years, including dielectric powder and magnetic ferrite powder. For example, as dielectric powder, barium titanate and titanium oxide excel in dielectric properties, piezoelectric and pyroelectric properties and are used as materials for ceramic capacitors, filters and sensors.
When using ceramic powder for ceramic capacitors, those with high dielectric properties and small losses are desirable. When using these powders for magnetic ferrite material, the desirable types are those with small losses and magnetic permeability that extends flatly to the high frequency range.
The characteristics of powder depend on the shape, particle size, purity and reactive properties of the ceramic powder. For example, if the ceramic powder is the polycrystalline type or comes in indeterminate shape, they could cause localized abnormal growth or uneven composition, inviting deterioration of magnetic characteristics or electrical characteristics. Therefore it is preferable that the ceramic powder has no crystal particle boundary or impurities, but is single-phase and single crystal. Moreover, in order to further acquire superior characteristics, it is preferable for the ceramic powder to be a compound of two or more types of metals and oxygen.
However, it is difficult to manufacture ceramic powder with superior characteristics. For example in solid-phase reaction method, it is possible to obtain a metal oxide dielectric compound of more than two types of metal and oxygen by firing the mixed powder of metal oxide within the atmosphere or in inert gas in accordance with the composition of the final formed product. However, it is difficult to obtain a single-phase powder. Also, in liquid phase methods, including co-precipitation method, the precursor (primary particle) of the metal oxide such as hydrates is made from the aqueous solution or organic solvent solution of metallic salts. This precursor is fired in the atmosphere or inert gas environment to create ceramic powder. However, it is difficult to obtain a dielectric powder with excellent crystallinity. Moreover, because the bonding of the metal oxide precursor is ultimately obtained as a large mass, it is necessary to pulverize the dielectric substance after the firing to obtain powder with dielectric properties. In powders obtained in such a manner, the shapes of individual particles are irregular, and the distribution of particle size will become expansive and there is a strong possibility of impurities becoming mixed in.
Therefore, while the hydrothermal synthesizing method and gas phase reaction method that improve the shape of the powdered particles and particle size distribution have been proposed, it will be difficult to manufacture the powder industrially in terms of productivity and cost. Also, a method has been proposed to obtain single crystal powder with uniform particle size by forming minute oxide particles from raw materials melted in solvents by using hydrolytic decomposition and co-precipitation method, crystallizing and growing particles after heat treating the fine powder, and dissolving and eliminating the glass included in the resulting product.
However, this method requires complex processes, and will render it difficult to produce the powder industrially.
Moreover, according to another method, barium titanate single crystal is obtained by sintering barium titanate with a mean particle size of less than 10 μm at temperatures ranging from 1,200° C. to less than 1,618° C. Under this method, barium titanate single crystal is formed by causing abnormal crystal grain growth of barium titanate by sintering the compound at temperatures lower than its melting point and by giving a temperature gradient during the sintering process. However, this method produces large size barium titanate particles of about 500 μm, and cannot produce fine particles. Moreover, in this method, the single crystals are obtained within polycrystalline particles, and therefore an additional process is needed to immerse the polycrystalline particles in concentrated hydrochloric acid to destroy the polycrystalline parts to obtain the single crystals.
While ceramic powders are used alone in their powder form, they are also used as composite materials with the powder dispersed in resin material. Ceramic powder used as composite material requires dispersion and filler-type properties for resin materials. One of the elements for securing dispersion and filling properties for resin material is the particle size of the fine particles comprising the powder. However, in the ceramic powder obtained through the aforementioned co-precipitation method, the dispersion and filler properties cannot be secured for resin material because the particles are too fine. Also, as for ceramic powder obtained through the liquid phase method described earlier, the shape of the powdered particles is irregular as the powder was obtained through pulverization, and therefore, the dispersion and filler properties for resin material cannot be secured. Moreover, as for the barium titanate single crystal obtained by the second conventional method described above, it would be difficult to obtain a high degree of filler property because particle size would be too large.